In a legged mobile robot such as a biped mobile robot equipped with a plurality of legs, each leg is brought into contact with a floor through a ground-contacting face portion of a foot mechanism provided on a far end portion thereof. More particularly, the foot mechanism is a mechanism connected to a joint on the farthest end side of each leg (an ankle joint). The legged mobile robot moves by lifting and landing motions of each leg. More particularly, the lifting and landing motions are a repetition of motions that while at least one leg of a plurality of legs as a supporting leg maintains a foot mechanism of the supporting leg in a ground-contacting state, the other leg as a free leg lifts a foot mechanism of the free leg from its ground-contacting location into the air and moves the same, and make contact with the ground on other ground-contacting location.
In such a legged mobile robot, when a ground-contacting face portion of a foot mechanism of the leg is brought into contact with the ground by the landing motion of each leg, a relatively great impact load (a transient floor reaction force) instantaneously acts through the foot mechanism of the leg. Particularly, when the legged mobile robot is moved at relatively high moving speed, motion energy of the leg in moments immediately before the foot mechanism of the leg makes contact with the ground is great, so that the impact load will be high. When this impact load is high, rigidity of each portion of each leg needs to be enhanced in order to resist the load, and furthermore, this will interfere with a size reduction and a weight reduction of each leg. Accordingly, a reduction (shock absorption) of such an impact load is desired.
As such a shock absorber, for example, the one that the present applicant proposed in Japanese Patent Laid-Open Publication No. 5-305578 is known. In this shock absorber, a cylinder filled with hydraulic oil is provided at a heel of the foot mechanism, and a rod is extendedly provided from a piston slidable in this cylinder toward a bottom face side of the heel of the foot mechanism. A ground-contacting element widened in diameter in a mushroom shape is provided on a tip portion of the rod. Additionally, the piston is energized in a direction that the ground-contacting element projects to the bottom face side of the foot mechanism by a spring accommodated in the cylinder on the upper side thereof. Furthermore, in the piston, a flow passage that allows the hydraulic oil to flow between an upper chamber and a lower chamber thereof is drilled.
In the shock absorber configured in this manner, at the time of the landing motion of the leg, the aforementioned ground-contacting element makes contact with the ground and is pressed with the piston in a direction opposite to an energizing force of the spring. At this moment, while the hydraulic oil in the cylinder flows through the flow passage of the piston, the piston slides in a direction that the piston compresses the spring, and this allows the impact load during the landing motion of the leg to be reduced.
However, in the shock absorber, as a result of the use of the hydraulic oil, particularly, when a moving speed of a robot is fast, pressure of the hydraulic oil suddenly increases at the instant when the ground-contacting element touches the ground. Accordingly, a relatively high impact load tends to be generated at the instant when the ground-contacting element touches the ground. In this situation, when an area of an aperture of the flow passage of the piston is designed to be relatively large, the sudden increase in the pressure of the hydraulic oil at the instant when the ground-contacting element touches the ground may be controlled. However, in a situation like this, a damping effect caused by a flow of the hydraulic oil (an attenuating effect of motion energy) decreases and a vibration of a floor reaction force immediately after the landing motion of the foot mechanism tends to be generated, and resultingly, a posture of the robot tends to be unstable.
Furthermore, in the shock absorber, as a result of the use of the hydraulic oil, weight of the shock absorber will be heavy, resulting in interfering with a weight reduction of the robot. Additionally, the ground-contacting element that makes contact with the ground during the landing motion of the leg can only move in a sliding direction of the piston (an axial center direction of the cylinder) and is a solid body. Consequently, the impact load acting on the ground-contacting element may act in a direction that crosses a movable direction thereof depending on a geometry of a floor, so that the impact load may not adequately be reduced, and a damage of the shock absorber may be generated.
In light of such a background, it is an object of the present invention to provide a landing shock absorber that can smoothly reduce an impact load during a landing motion of a leg of a legged mobile robot with a light-weight configuration.